Network ring topologies are gaining in popularity, particularly in Internet Protocol (IP) networks. Such networks enable carriers to offer large bandwidth to users in a cost-effective manner, since each node in the network needs only two interfaces, rather than having to maintain a separate interface for each of the other nodes as in a mesh network.
Recently-developed bidirectional protocols provide efficient bandwidth utilization by enabling data to be transferred between any pair of nodes in either direction around the ring, while maintaining fast protection against faults. The two opposing traffic directions are commonly referred to as an inner ring and an outer ring. It will be understood, however, that in the context of the present patent application and in the claims, the terms “inner” and “outer,” as well as “clockwise” and “counterclockwise,” are used arbitrarily to distinguish between the two opposing directions of packet flow in a ring network. These terms are chosen solely for convenience of explanation, and do not necessarily bear any relation to the physical characteristics of the network.
The leading bidirectional protocol for high-speed packet rings is the Resilient Packet Rings (RPR) protocol, which is in the process of being defined as IEEE standard 802.17. Network-layer routing over RPR is described, for example, by Jogalekar et al., in “IP over Resilient Packet Rings” (Internet Draft draft-jogalekar-iporpr-00), and by Herrera et al., in “A Framework for IP over Packet Transport Rings” (Internet Draft draft-ietf-ipoptr-framework-00). A proposed solution for Media Access Control (MAC—protocol layer 2) in bidirectional ring networks is the Spatial Reuse Protocol (SRP), which is described by Tsiang et al., in Request for Comments (RFC) 2892 of the Internet Engineering Task Force (IETF). These documents are incorporated herein by reference. They are available at www.ietf.org.
Using protocols such as these, each node in a ring network can communicate directly with all other nodes through either the inner or the outer ring, using the appropriate Media Access Control (MAC) addresses of the nodes. Each packet sent over one of the rings carries a header indicating its destination node. The destination node recognizes its address in the header and strips the packet from the ring. All other nodes pass the packet onward transparently around the ring.
It is common in all sorts of data communication networks for network users to enter into a service level agreement (SLA) with their network service providers. The SLA specifies the quality of service (QoS) that the user is entitled to receive, based on a list of service parameters, such as network availability, percent of packets lost, mean latency of packet delivery, and deviation of the latency relative to the mean. Controlling latency is important particularly for real-time services, such as voice and video. In order to commit to a SLA that includes latency-related parameters, the service provider must be able to monitor the latency so as to verify compliance with the SLA.
Methods of network latency measurement are known in the art. For example, U.S. Pat. No. 5,235,593, to Grow et al., describes a ring latency timer for the purpose of latency measurement in token rings. A token ring is a closed, unidirectional ring network. A small token frame circulates around the ring. A node wishing to transmit on the ring captures the token and is then granted control of the transmission medium for a specified period. After capturing the token, the node transmits an information frame on the ring. When one of the nodes recognizes the destination address of the frame as its own, it copies the frame. After completing the transmission, the sending node releases the token for use by the other nodes.
In order to measure the ring latency, the node sends a special “void” frame before releasing the token. The void frame contains the address of the sending node in both the sender and destination fields. A latency counter is initialized when the void frame is released, and it is incremented every 16 clock cycles until the void frame is received back at the sending node. A latency bit is then set, and the value of the counter indicates the latency of the ring. The value remains valid until the latency bit is reset, whereupon the measurement is repeated.
The measured latency value gives an indication of the traffic load on the ring. The measured value is compared to a target token rotation time (TRTT) in order to determine whether and when each of the nodes in the ring is entitled to transmit data. Different TRTT values can be set for different classes of service on the ring, in order to allocate ring bandwidth preferentially to high-priority services. All of these TRTT values are compared against the same measured latency value to determine when the different services are allowed to transmit frames on the ring.
The methods described by Grow et al. can be used for measuring latency of transmission around the entire ring in a single direction, but they do not provide a means for measuring round-trip, node-to-node latency between a pair of nodes on the ring. Most methods known in the art for measuring such round-trip latency are based on measuring the time required for a packet to be sent from a source node to a destination node and then back again. These methods suffer from the drawback that the measured latency includes the time taken by the destination node to process the packet that it receives from the source node and to generate the response packet to send back to the source node.
Alternatively, it is possible to measure one-way latency from the source node to the destination node, and thus to eliminate the effect of the processing time at the destination node. Such a measurement, however, requires precise synchronization between the clocks at the source and destination nodes. Most networks do not maintain sufficiently accurate synchronization between nodes to enable meaningful measurements of one-way latency to be made. Therefore, costly hardware must typically be added to the network to enable accurate one-way latency measurements.